1. Field of the Invention
The present invention relates generally to a non-invasive oximeter capable of measuring arterial oxygen saturation, and more particularly, to an optical oximeter which analyzes light wave signals transmitted through a living body to detect the oxygen saturation in the arterial blood contained therein.
2. Description of the Prior Art
In general, methods for measuring oxygen saturation in arterial blood without penetrating body tissue utilize the relative difference between the light absorption coefficient of hemoglobin (Hb) and that of the hemoglobin oxide (HbO.sub.2). The light absorption coefficient for Hb and HbO.sub.2 is characteristically tied to the wavelength of the light traveling through them. Both Hb and HbO.sub.2 transmit light having a wavelength in the infrared region to approximately the same degree. However, in the visible region, the light absorption coefficient for Hb is quite different from the light absorption coefficient of HbO.sub.2.
One example of a non-invasive oximeter is described in an article titled "Photoelectric Determination of Arterial Oxygen Saturation in Man" by Wood and Geraci, in the Journal of Laboratory and Clinical Medicine, Volume 34, 1949. The oximeter described therein utilizes a light source that generates light in the infrared region and in the red region. Both light wave signals are transmitted through body tissue. The respective light wave signals leaving the body tissue are photoelectrically converted into a first and second output signal. Ultimately, these signals are analyzed to get an indication of the oxygen saturation in the arterial blood. Before these first and second signals are generated, the body tissue is compressed to occlusive pressure (200 mm. of mercury), squeezing blood from the tissue under test. This is done to obtain reference output signals which have information regarding light absorption by the tissue itself, i.e., muscles, bone or skin, without the blood. These reference signals are required in order to separate the information regarding light absorption by blood alone from the above mentioned first and second output signals, which includes information regarding light absorption by the blood and tissue together.
As a result of this tissue compression set up, it is impossible to continuously detect the oxygen saturation in the blood, because a measurement under compression interrupts the normal flow of blood. As a result of the necessity of providing a compression pressure of 200 mm. of mercury, this prior art oximeter is quite bulky and cumbersome, and may even introduce a certain amount of discomfort to the subject.